In each of the above examples, the air spring comprises essentially two variably mutually spaced end members, namely, a cover and a roll-off piston and a flexible member clamped pressure tight therebetween, especially, a rolling-lobe flexible member.
In publication DE 100 25 631 A1, a method is described wherein the height of the spring is determined by means of the high frequency hollow space resonance. The flexible member must have good conductivity so that the flexible member performs as an electromagnetic hollow space resonator. This can, for example, be achieved in that the reinforcements, which are introduced into the flexible member, are electrically conductive.
This publication emphasizes details of the measuring electronics. Details as to the configuration of the electrically conductive reinforcements are not disclosed.
According to DE 100 17 562 C1, the measurement of height takes place with the aide of two coils one of which is mounted axially secure within the air spring interior space and the other one of the coils is mounted between the cover and the roll-off piston so as to be changeable in length. A level dependent measurement signal results because of the change of the height position of the air spring as well as because of the compression operation. The length-changeable coil can be an integral component of the flexible member, that is, of the wall. This coil is either pressed onto the surface of the flexible member facing inwardly or is glued or is worked directly between the layers.
An application of the coil of this kind on or in the wall of the flexible member requires an additional work step in the production of the flexible member or in the production of the air spring. Problems can develop with the flexibility of the wall of the flexible member (harshness effect) because the coil is not mounted in the plane of the fabric ply or plies.
The flexible members of the air springs, described in publications DE 40 35 784 A1 and DE 44 13 559 A1, likewise show measurement fibers worked into the wall.
It is, however, the case that according to DE 40 35 784 A1, electrical conductors are worked into the wall of the flexible member in the form of a coil or diagonally. Here, the conductor paths are configured as a coil to be changeable in length with the coil being applied to a latex monofil. The incorporation of latex monofil fibers, which are provided with electrically conducting coils, into the wall of the flexible member is likewise associated with additional work complexity in the manufacture.
According to DE 44 13 559 A1, the electrically conductive measuring fibers, which are integrated into the wall of the flexible member, are characterized by running parallel to the fiber direction of a fabric ply and in the longitudinal direction of the flexible member from one flexible member end to the other. Because of the position and the arrangement of these conductor paths, their inductivity changes with the spring height because of the spring operation.
The fibers, which are to be introduced into the wall of the flexible member, comprise, for example, copper strands which must be introduced into the wall of the flexible member in addition to the textile fabric plies or in lieu of individual fibers of the fabric plies. If the copper fibers are not arranged in the plane of the textile reinforcement, then there results overall a stiffening of the wall of the flexible member and the consequences are a pronounced harshness effect. If the copper fibers are in the plane of the textile reinforcement, then there results an inhomogeneous expansion of the wall of the flexible member during loading because of the different expansion characteristics of the copper strands and textile cords whereby the service life of the flexible member is affected.